Japanese Patent Application Publication No. HEI 4-219553 discloses an automatic transmission that is provided with a simplex planetary gear set including a first ring gear coupled to an input shaft, a first sun gear fixed to the transmission case, and a first carrier that supports pinions that mesh with the first ring gear and the first sun gear, and a duplex planetary gear set having second and third sun gears, long pinions that directly mesh with the second sun gear and mesh with the third sun gear via intermediate pinions, a common carrier that supports the long pinions and the intermediate pinions, and a common ring gear that meshes with the long pinions. The common ring gear of the duplex planetary gear set is directly coupled to the output shaft, and the rotation of the first carrier of the simplex planetary gear set, reduced in rotational speed from that of the input shaft, is selectively transferred to the third and second sun gears of the duplex planetary gear set via first and third clutches, while the rotation of the input shaft is selectively transferred to the carrier of the duplex planetary gear set via a second clutch. The second sun gear and the common carrier of the duplex planetary gear set are selectively fixed via first and second brakes, respectively, thereby establishing six forward shift speeds and a reverse speed.
U.S. Pat. No. 5,542,889 describes, as shown in FIG. 1, an automatic transmission having a simplex planetary gear set 52 including a carrier 74 directly coupled to an input shaft 44, which is rotationally coupled to a crankshaft 14 of an engine, a sun gear 70 fixed to a transmission case 80, and a ring gear 72 that is rotated at a speed higher than that of the input shaft 44, and a duplex planetary gear set 50, 48 including a ring gear 64 and a carrier 58 which are directly coupled, and a carrier 66 and a ring gear 56 which are selectively connected via a clutch CL1. A sun gear 62 in the duplex planetary gear set 50, 48 is selectively connected to the carrier 74 of the simplex planetary gear set via a clutch CL3, or is fixed to the transmission case 80 via a brake B2. Reverse rotation of the carrier 66 is prevented by a one-way clutch 82. Reverse rotation of the ring gear 56, coupled to the carrier 66 via the clutch CL1, is prevented by a one-way clutch 82. The carrier 66 is selectively connected (coupled) to the ring gear 72 of the simplex planetary gear set 52 via the clutch CL2, and is selectively fixed to the transmission case 80 via a brake B1. The directly connected ring gear 64 and carrier 58 are directly coupled to an output shaft 90, and a sun gear 54 of the planetary gear set 48 is directly coupled to the input shaft 44.
The automatic transmission described in Japanese Patent Application Publication No. 2000-220704 is similar to that described in Japanese Patent Application Laid-Open Publication No. HEI 4-219553, but includes an input shaft 11 which is rotationally driven by an engine and which is divided into a forward half portion 11A and a rearward half portion 11B, which portions are spline-joined as shown in FIGS. 6 and 7. An oil passage 11p for supplying operating oil to a hydraulic servo 6, and a lubricating oil passage 11r for supplying lubricating oil to the elements disposed in a transmission case 10 are formed in parallel in the forward portion 11A. A lubricating oil passage 11s is formed in the forward portion 11B. The carrier C1 of simplex planetary gear set G1 is not coupled to the input shaft 11, but is coupled to the common hub 74 of first and third clutches C-1, C-3.
In the automatic transmissions described in Japanese Patent Application Publication No. HEI 4-219553 and Japanese Patent Application Publication No. 2000-220704, the output rotation of the engine is reduced in speed by the simplex planetary gear set and then input to the duplex planetary gear set. Therefore, an increased torque is transferred to the duplex planetary gear set. In particular, if the engine outputs high torque at relatively low speeds as in, for example, diesel engines, the transferred torque becomes even larger, and therefore it is necessary to increase the sizes of various components and to increase the capacities of clutches and brakes in order to cope with the high torque.
In order to solve the above-described problem, the automatic transmission described in of U.S. Pat. No. 5,542,889 increases the output speed of rotation of the engine by transfer through a simplex planetary gear set 52 and then through a duplex planetary gear set 48, 50. However, in the automatic transmission described in U.S. Pat. No. 5,542,889, the carrier 66 is prevented from reverse rotation by the one-way clutch 82, and the ring gear 56 is coupled to the carrier 66 by the clutch CL1 and is therefore prevented from rotating, so that the rotation of the input shaft 44 directly coupled to the sun gear 54 is reduced in speed, thereby achieving the first speed. Therefore, this automatic transmission is complicated in structure. In addition, in order to prevent rotation of the ring gear 56 that receives the reaction force for the first speed, where the transfer torque is large, there is a need to increase the capacity of the clutch CL1 that couples the ring gear 56 to the one-way clutch 82, resulting in an increased size of the apparatus.